1. Field of the Invention
This invention relates to new and useful improvements for electrical cables adapted for use in supplying electrical power and communications and, more particularly, to an improved corrosion resistant cable shielding tape forming a part of such cables. More specifically, the present invention relates to cable shielding tapes comprising a relatively thin metal strip with one or more layers of polymeric resinous material adhered to at least one side thereof.
2. Description of the Prior Art
In the art of designing and constructing electrical cables, especially telecommunication cables such as telephone cables, it is known to assemble insulated conductors in a core and surround it by shield and jacket components. A well known telephone cable design of such construction is referred to in the art as an "Alpeth" cable. This type of cable is more fully described by F. W. Horn et al. in the paper "Bell System Cable Sheaths Problems and Designs" in A. I. E. E. Proceedings 1951, Volume 70. The shielding tape of the "Alpeth" cable is formed of a layer of bare aluminum having a thickness of about 8 mils which is usually corrugated transversely prior to being wrapped about the cable core. The corrugations impart greater flexibility to the cable and permit bending of the cable without wrinkling or rupturing of the shielding tape.
The term "shield, screen, or shielding tape" as used herein means a relatively thin layer of any metal, bare or coated, which can provide mechanical protection and electrostatic and electromagnetic screening for the conductors in the core of electrical power and communication cables.
When telephone cables are installed underground by being buried directly in soil, the outer jacket of such cables, which is formed of a polymeric resinous material such as polyethylene, may be subjected to damage due to the rigors of installation, and by rocks, rodents, lightning, frost, or dig-ins following the installation. The underlying shielding tapes can thereby be exposed to sub-surface water and the attendant potential for corrosion.
Where the outer jacket of such cables is formed from a polymeric resinous material such as polyethylene, the jacket is not well adhered to the shielding tape of bare metal. The outer plastic jacket is known to slip over the shielding tape and to fold up into shoulders as the cables are pulled through ducts or placed into trenches. The shielding tape is also known to kink, curl or twist during installation causing metal fatigue in the tape and, in extreme cases, rupture of the tape because of mechanical bending stresses exerted thereon.
In order to improve the corrosion resistance of a shielding tape of bare metal, a special adhesive polyethylene coating may be applied to cover one or both sides of the metallic strip as taught in U.S. Pat. Nos. 3,233,036 and 3,795,540. Such shielding tapes are widely used in the manufacture of electrical power and communications cables. The adhesive polyethylene used for this film contains reactive carboxyl groups which have the ability to develop firm adhesion to the metallic strip and also to the overlying polyethylene jacket. The metal component of such shielding tapes provides electrostatic screening and mechanical strength to the cable; the coating of the polymeric resinous material, e.g., ethylene acrylic acid (EAA) copolymer, provides bondability, sealability and corrosion protection to the metal component.
When a polyethylene jacket is extruded over the metallic strip coated with the adhesive polyethylene film, the heat from the semi-molten polyethylene jacket bonds the film coated metal strip to the jacket, forming a unitized construction which combines the strength of the metal strip component with the elongation and fatigue resistance of the polyethylene jacket component. Such cable constructions are referred to in the art as "Bonded Jacket" cable designs. If the heat content of the extruded polyethylene is sufficiently high, the shielding tape could become hot enough so that the overlapped portions of the shielding tape bond together at the seam, thereby forming a sealed tube or pipe around the core of the cable. The "Bonded Jacket" cable with a sealed seam has improved resistance to moisture penetration into the cable core. This cable construction also has been shown to have the greater mechanical strength necessary to withstand repeated bending of the cable, i.e. reducing the occurrences of kinking and fatigue failures of the shielding tape resulting from bending stresses during installation. Further, the effects of stresses induced by the temperature cycles experienced by cables under service conditions are reduced.
The thin plastic coating protects the underlying metal to some degree from corrosion by limiting the area over which such corrosion can occur or by preventing contact between the metal and the sub-surface water. The coating should be tightly bonded to the metal to resist significant delamination therefrom during exposure to the corrosive water and the mechanical forces exerted by the formation of voluminous metal corrosion products, thereby restricting the path of corrosive attack to the exposed metal edges of the shielding tape.
Recently, examination of several commercial cables utilizing polymer resinous material coated shielding tapes representative of the prior art has revealed, however, that the coatings on such tapes are damaged during cable manufacture exposing numerous corrodible bare spots on the surfaces of the metal strip. More specifically, when a polyethylene jacket is extruded over a plastic coated metal shielding tape, the heat from the molten polyethylene jacket softens or melts the polymeric resinous material coating to obtain a bond to the jacket and a sealed seam. While the coating on the metal tape is in such softened or molten state, it is penetrated or abraded by the smooth, corrugated or embossed core wraps, by the seams of the tape itself, by the binder tapes, and/or by the weight of the core itself, thereby exposing numerous corrodible bare spots on the surfaces of the metal strip. As a result, the corrosion rate at the damaged spots may be accelerated due to an unfavorable ratio of the anodic and cathodic areas of bare and coated metal. Furthermore, corrosion which propagates circumferentially between damaged spots may prematurely destroy the longitudinal continuity of the shielding tape which, in turn, could render the cable inoperative. Since telephone cables are expected to have a long service life, corrosion of shielding tapes which can lead to premature cable failures is indeed a serious technical and financial problem for the wire and cable industry. The problem of coating damage has not been recognized until the present invention because of the industry's preoccupation with other major technical design problems. One such problem was the need to develop thermal barrier materials to protect the cable core from heat damage. Another problem was associated with the introduction of fully-filled telephone cable designs wherein the cable core is filled with a grease-like compound to prevent ingress and migration of water.
The corrodible bare spots may occur on either side of the shielding tape but the problem is particularly critical with the use of corrugated metallic strips where it has been observed that the penetration and/or abrasion damage exposing the bare metal is concentrated on the raised corrugated surfaces of the shielding tape disposed toward the core. A corrosive attack on this type of circumferentially concentrated damaged area on the corrugated metal strip will quickly destroy the longitudinal electrical function of the shielding tape. In order to maintain the prior art criterion of restricting corrosion to the shielding tape edges, it is now recognized that penetration and/or abrasion resistance of the plastic coatings is required, in addition to delamination resistance, to insure that corrosion is generally confined to the edges of the shielding tape instead of being extended over the entire surface thereof.
Although there is no known prior art directly concerned with overcoming the above identified problems, the following prior patents specifically referred to hereinbelow and in Table II illustrate the closest known prior art in the plastic coated shielding tape technology.
U.S. Pat. No. 3,586,756 and U.S. Pat. No. 3,950,605 (Example 3 and 6--Table II) disclose shielding tapes comprising a metal strip having an adhesive polymer coating adhered to at least one side of the metal strip. However, these prior patents do not provide for a deformation resistant layer of a polymeric resinous material composition having a deformation temperature of at least 270.degree. F. The coating on such tapes will be deformed during cable manufacture exposing numerous corrodible bare spots on the surfaces of the metal strip.
U.S. Pat. No. 3,507,978 (Example 4--Table II) teaches a shielding tape comprising a metal foil having layers of a copolymer such as ethylene/acrylic acid chemically bonded to both sides of the metal foil and an additional layer of high density polyethylene bonded to one of the copolymer layers. However, there is no teaching or suggestion in U.S. Pat. No. 3,507,978 of the damage problem overcome by the present invention and examination of commercial cables incorporating such a shielding tape also illustrates that penetration and/or abrasion of the high density polyethylene layer occurs at current cable manufacturing conditions.
U.S. Pat. No. 3,379,824 (Example 8--Table II) teaches a shielding tape comprising a three layer structure with an aluminum foil laminated between two polypropylene layers or a polypropylene layer and a polyethylene terephthalate layer. Again, there is no teaching or suggestion of the damage problem overcome by the present invention. In addition, although these plastic layers will resist penetration and abrasion, they do not provide corrosion protection when a corrosive environment is present in a cable since both polypropylene and polyethylene terephthalate are highly inert and can develop only a poor mechanical bond to the metal strip based on friction adhesion. Therefore, both the polypropylene and polyethylene terephthalate layers will easily delaminate under exposure to corrosive conditions and the mechanical forces exerted by metal corrosion products. Furthermore, neither polypropylene nor polyethylene terephthalate can achieve the bonded jacket and sealed seam.
U.S. Pat. No. 3,325,589 (Example 9 to 11--Table II) discloses a plastic coated metal shielding tape comprising a metal strip having an adhesive layer immediately adjacent to the metal strip and an additional polyester or polypropylene layer adhered to one side of the metal strip. Such a shielding tape was subjected to simulated conditions of cable manufacture and a laboratory corrosion test. It was found that the tape did not provide satisfactory corrosion resistance to the metal, i.e., the path of corrosive attack was not confined to the exposed metal edges. The adhesive layer was deformed from pressure exerted through the polypropylene or polyester layer thereby exposing bare aluminum spots. Corrosion was taking place on these bare spots after subjecting the cable to a standard corrosion test with sodium hydroxide (NaOH), as hereinafter defined in this specification, due to the infiltration of the NaOH between the adhesive layer and the polypropylene (PP) or polyester layers.
U.S. Pat. No. 3,790,694 (Example 9--Table II) discloses a polypropylene layer adhesively bonded to a metal strip. The patent does not specify the use of any particular adhesive. Therefore, ethylene acrylic acid (EAA) copolymer, the best known metal adhesive in the industry today, was used to make shielding tapes according to the teachings of that patent. When subjected to simulated conditions of cable manufacture and a standard corrosion test with NaOH as hereinbefore defined in the specification, the shielding tapes were found to give similar results to those of U.S. Pat. No. 3,325,589: The EAA adhesive layer does not adhere tightly to the polypropylene layer thus allowing the infiltration of NaOH solution therebetween. The patent teaches bonding of the jacket, a screen, and composite tapes together during extrusion of the cable jacket. Since the thermoplastic coatings on the screen and composite tapes must be above their melting points to effect bonding, they were found to be damaged a priori. Thus, this prior art patent also failed to recognize the problem of coating damage on shielding tapes. U.S. Pat. Nos. 3,325,589 and 3,790,694, are related to a heat resistant core wrap (thermal barrier) and a fully filled cable, respectively.
U.S. Pat. No. 3,321,572 (Example 13--Table II) and U.S. Pat. No. 3,622,683 (Example 8--Table II) disclose, inter alia, shielding tapes comprising a metal strip having a polymeric resinous material coating adhered to at least one side thereof and capable of resisting deformation at an elevated temperature. However, these shielding tapes were found to fail the adhesion requirement of the present invention. In these tapes, it was found that the path of corrosive attack was not confined to the exposed edges of the metal strip because of the infiltration of the corrosive element between the polymer coating and the metal strip.
U.S. Pat. No. 3,484,539 teaches the adhesion of a heat sealable layer, such as, for example, polyvinyl chloride to a polymer layer capable of resisting deformation at cable-forming temperatures. However, the polymer layer of this patent, having adhered thereto a heat sealable layer, it not "tightly bonded" to the metal strip and is thus open to corrosive attack due to the infiltration of corrosion causing liquids when the cable jacket is damaged.